Ages 4–8
At a certain age children love to know secrets, and Valentine’s Day lets them show it. An example: The following two out-of-copyright lines appear in “To a Baby Boy,” collected in Songs and Other Verse, by the American children’s poet Eugene Field (1850–1895):

Who I am I shall not say,
But I send you this bouquet

Children could attach the lines to a bouquet — or to a drawing (or sticker collage) of flowers — for an easy-to-make free card.

Ages 9–12
Tweens and older children may be embarrassed by overtly romantic sentiments, yet still want or need to send Valentine’s Day cards. The complete works of the Elizabethan poet Michael Drayton (1563–1631) include four out-of-copyright lines that might be sufficiently neutral:

Teenagers
Teenagers who believe they are desperately in love can find many appropriate poems online. Among the best-known: Elizabeth Barrett Browning’s iambic pentameter sonnet “How Do I Love Thee?”(Sonnet 43), which begins:

How do I love thee? Let me count the ways.
I love thee to the depth and breadth and height
My soul can reach, when feeling out of sight
For the ends of being and ideal grace.

This much-parodied poem has become a cliché to many adults but doesn’t sound trite to teenagers hearing it for the first time (and might inspire some to have fun writing their own parodies). The same goes for the lyrics to that Beatles’s toe-tapper, “When I’m Sixty-Four”:

When I get older losing my hair,
Many years from now.
Will you still be sending me a valentine
Birthday greetings bottle of wine.

You can hear Paul McCartney singing this one on YouTube by searching for “When I’m Sixty-Four” + “lyrics” (though I’m not linking to it because I’m not convinced that any versions of the song on YouTube are legal).

Other good poems and ways to celebrate the day appear in Ann Heinrich’s Valentine’s Day: Holidays, Festivals, & Celebrations (The Child’s World, 2006), illustrated by Sharon Holm, and other books.